The present invention relates generally to an ocular optical system, and more particularly to an ocular optical system intended for use with a head- or face-mounted image display device which is mounted on the head or face of the user to project images into the user's eyeballs.
In recent years, helmet or goggle-type of head-mounted or face-mounted image display devices have been developed for virtual reality purposes or with a view to allowing individuals to enjoy wide-screen viewing.
For instance, JP-A-2-297516 discloses an optical system made up of a 2D display device 11 for displaying images, an objective collimating lens 12 and a parallel transparent plate 13 having off-axis paraboloidal mirrors on both its ends, as shown in FIG. 6. Light beams leaving the 2D display device 11 for displaying images are converted by the objective collimating lens 12 into parallel beams, which are then subjected to first transmission through a parallel surface of the parallel transparent plate 13, reflection at the first paraboloidal mirror, some total reflections within the parallel transparent plate 13, reflection at the second paraboloidal mirror and second transmission through the parallel surface of the parallel transparent plate 13 (8 reflections and 2 transmissions), whereby an intermediate image is formed on a point F for projection into an observer's eyeball 14.
U.S. Pat. No. 4,026,641 discloses an optical system wherein, as shown in FIG. 7, a image of an image display element 11 is converted by a transmission optical element 15 into a curved object image, which is in turn projected from a toric surface 16 into an observer's eyeball 14.
A problem associated with an image display device of the type wherein an image thereof is relayed as shown in FIG. 6 is, however, that it needs not only an ocular optical system, but also a relay optical system, resulting in increases in the size and weight of the whole optical system as well as an increase in the amount of extension of the whole optical system from a person's head or face. Therefore it is not fit for a head-mounted or face-mounted image display device.
The optical system for forming parallel beams as an intermediate image as well as the optical system for projecting an intermediate image into an eyeball produce some considerable aberrations because only the paraboloidal mirrors that have power.
The ocular optical system composed of a concave mirror, as shown in FIG. 7, also produces some considerable aberrations and so is detrimental to image quality, even though the concave mirror is defined by a toric surface as shown in FIG. 7.
To correct for field curvature occurring at the ocular optical system, therefore, it is required to use the transmission optical element 15 such as a fiber plate. Even by use of the transmission optical system 15 and toric surface 16, however, it is impossible to make adequate correction for coma, and other aberrations.